1. Field of the Invention
The present invention relates to a high zirconia fused cast refractory suitable for use as a refractory for a glass tank furnace.
2. Discussion of Background
Fused cast refractories are obtained by charging blended refractory materials into an electric arc furnace, completely melting them, then pouring the meltage into casting molds of prescribed shapes, followed by cooling to room temperature for solidification usually under thermal insulation. They are widely known as refractories which are dense and excellent in corrosion resistance and which are entirely different in the structure and the process for their preparation from fired or unfired bonded refractories.
Among such fused cast refractories, those containing a large amount of ZrO.sub.2 exhibit particularly excellent corrosion resistance against molten glass. Therefore, zirconia fused cast refractories are widely used at the wall portions of a glass tank furnace which are in contact with molten glass.
It is known, however, that high zirconia fused cast refractories wherein the major structure is composed of ZrO.sub.2 crystals (baddeleyite), undergo a reversible crystal phase transformation from monoclinic crystals to tetragonal crystals specific to ZrO.sub.2 crystals, at about 1,100.degree. C., and they undergo anisotropic volume expansion or shrinkage due to such a crystal phase transformation, whereby it is extremely difficult to obtain practically useful crack-free fused cast refractories of large sizes.
Heretofore, a variety of fused cast refractories containing 90% by weight or more of ZrO.sub.2 and a matrix glass which is composed essentially of SiO.sub.2 and which fills spaces among ZrO.sub.2 crystals have been proposed. However, the main measure to prevent formation of cracks is a method of incorporating a component for softening the matrix glass to adjust the viscosity of the matrix glass, so that the strain due to the expansion or shrinkage of the ZrO.sub.2 crystals within the temperature range for the crystal phase transformation of the ZrO.sub.2 crystals, will be absorbed by the soft matrix glass, thereby to avoid formation of cracks.
For example, Japanese Examined Patent Publication No. 3319/1980 proposes to adjust the viscosity of a matrix glass by incorporation of a CuO or B.sub.2 O.sub.3 component which softens the matrix glass which is composed mainly of SiO.sub.2 and contains Al.sub.2 O.sub.3. Here, the Al.sub.2 O.sub.3 /SiO.sub.2 ratio (weight ratio, the same applies hereinafter) is specified to be less than 0.5, since if the Al.sub.2 O.sub.3 /SiO.sub.2 ratio is 0.5 or higher, the matrix glass can not adequately be softened. Moreover, when a refractory containing a CuO component, is used for a wall of a glass tank furnace, there will be a problem of coloring glass. Accordingly, such a refractory is not suitable for a glass tank furnace for melting glass which is desired to be colorless, such as ordinary sheet glass.
Further, Japanese Examined Patent Publication No. 12619/1984 proposes to absorb the expansion and shrinkage of ZrO.sub.2 crystals by softening a matrix glass which is composed mainly of SiO.sub.2 and contains an Al.sub.2 O.sub.3 component, by incorporating a P.sub.2 O.sub.5 component to the matrix glass. In this case, a crack-free fused cast refractory can be obtained even without adjusting the Al.sub.2 O.sub.3 /SiO.sub.2 ratio to a level of less than 0.5, and a refractory which is free from coloring glass and which is substantially free from forming stones (defects) in molten glass when used as a refractory for a wall of a glass tank furnace, can be obtained.
Japanese Examined Patent Publication No. 40018/1990 proposes a high zirconia fused cast refractory wherein the content of alkali metal oxides in the refractory is minimized to a level of not higher than 0.1% by weight to increase the electric resistivity within the service temperature range, and a P.sub.2 O.sub.5 component and a B.sub.2 O.sub.3 component are incorporated to soften the matrix glass and thereby to prevent cracking, so that the high zirconia fused cast refractory can be used for electrical glass melting.
Further, Japanese Unexamined Patent Publication No. 285173/1988 proposes to produce a fused cast refractory free from cracking and having a high electrical resistivity, by incorporating a B.sub.2 O.sub.3 component and a component such as K.sub.2 O, Rb.sub.2 O, Cs.sub.2 O, SrO or BaO which is an oxide of an alkali metal or an alkaline earth metal having a relatively large ion radius, to a matrix glass which is composed mainly of SiO.sub.2 and contains Al.sub.2 O.sub.3.
Meanwhile, with high zirconia fused cast refractories, chipping off of the surface layer of the refractories (chipping off phenomenon) within a temperature range of from 400.degree. to 600.degree. C. during the temperature rise, has been pointed out as a problem, and Japanese Unexamined Patent Publication No. 100068/1989 proposes to limit the contents of the P.sub.2 O.sub.5 component and the B.sub.2 O.sub.3 component to obtain high zirconia fused cast refractories free from the chipping off phenomenon.
Further, Japanese Unexamined Patent Publications No. 218980/1991 and No. 28175/1991 propose to form a matrix glass which comprises SiO.sub.2, Al.sub.2 O.sub.3, ZrO.sub.2 and Na.sub.2 O components and which does not contain P.sub.2 O.sub.5, B.sub.2 O.sub.3 or CuO and to incorporate prescribed amounts of the Al.sub.2 O.sub.3 component and the Na.sub.2 O component, to obtain fused cast refractories free from cracking and at the same time to prevent the chipping off phenomenon by a decrease or a change in quality of the matrix glass due to precipitation of crystals of e.g. zircon (ZrO.sub.2 .multidot.SiO.sub.2) in the matrix glass and to obtain fused cast refractories free from a tendency for an accumulation of remaining volume increase (having thermal cycle resistance).
By these proposals, it has been made possible to obtain refractories which have high thermal cycle resistance and high corrosion resistance against molten glass and which are free from contaminating molten glass and free from the chipping off phenomenon and have little blistering tendency (foam generating tendency). Such high zirconia fused cast refractories have now been widely used for glass tank furnaces. Further, high zirconia fused cast refractories having high electric resistivity suitable for electrical glass melting have recently been commercially available, and their applications are expected to expand to the field of special glass.
However, as a result of a creep test conducted under load at an elevated temperature using the high zirconia fused cast refractories proposed in Japanese Unexamined Patent Publications No. 218980/1991 and No. 28175/1991 as test samples and simulating the application to a wall of a glass tank furnace, a phenomenon has been observed wherein the matrix glass exudes from the refractory surface under load at an elevated temperature, and it has been deduced that when such refractories are practically used as refractories for a glass tank furnace under load at an elevated temperature for a long period of time, the molten glass will be contaminated by the glass exuded from the refractory surface, which causes formation of defects called codes in the glass product.